Generally known image-forming apparatuses include an electrophotographic copying machine or printer, and a toner-jet-type copying machine or printer. Such an image-forming apparatus generally includes a photoreceptor (e.g., a photoreceptor drum), which generates electric charge upon light exposure. In addition to the photoreceptor, the image-forming apparatus has a plurality of conductive members including conductive rollers such as a development roller, a charge roller, a transfer roller, and a toner-supplying roller, and a conductive belt such as a transfer belt.
One known conductive roller is a bi-layer configuration conductive roller having a core, an elastic layer which is made of conductive rubber and formed on the core, and a coating layer which protects the elastic layer and is formed on the elastic layer. Another known conductive roller is a tri-layer configuration conductive roller having, between the elastic layer and the coating layer, a resistance-adjusting layer which can control the electric resistance of the conductive roller and is formed of another conductive rubber.
Preferably, conductive rollers can uniformly retain a specific amount of toner on their roller surfaces. The amount of the toner maintained on the surface of the roller depends mainly upon the surface roughness of the roller or upon electric force attributed to the amount of charged toner. Thus, excellent image development characteristics (e.g., uniformity in image density) can be ensured by regulating the surface micro-roughness of the conductive roller, to thereby control the amount of toner transported to the conductive roller. For this purpose, there have been proposed a variety of techniques which provide appropriate micro-surface roughness of such rollers through incorporation of microparticles into the aforementioned coating layer.
For example, Patent Document 1 discloses a charge-imparting member as an example of the above techniques. The charge-imparting member includes a conductive support, a conductive elastic layer stacked on the conductive support, and a conductive resin layer stacked as the outermost layer on the conductive elastic layer. The conductive resin layer contains a matrix material and at least one particulate material selected from the group consisting of resin particles and inorganic particles, wherein the particulate material contains first particles. In the case where the thickness of the layer which is formed only of the matrix material and which is included in the conductive resin layer is represented by A [μm], the mean particles size of the particles is represented by B1 [μm], and the inter-particle distance is represented by Sm [μm], A is 1.0 μm to 7.0 μm, B1/A is 5.0 to 30.0, and Sm is 50 μm to 400 μm.
Patent Document 2 discloses an image-forming apparatus as another example of the above techniques. The image-forming apparatus includes a charge-imparting unit having an electrophotographic photoreceptor (positively charged type, single layer) and a contact charge-imparting member for electrifying the surface of the photoreceptor; a light exposure unit for irradiating the surface of the charged image on the support with light, to thereby form an electrostatic latent image on the surface of the support; a development unit for developing the electrostatic latent image, to thereby form a toner image; and an image transfer unit for transferring the toner image on the support to a transfer target. The contact charge-imparting member is a charge roller formed of a conductive rubber having a rubber hardness (Asker-C hardness) of 62° to 81°. The surface roughness of the charge roller of the contact charge-imparting member is defined by an average inter-groove (inter-gap) distance along a cross-section curve (Sm) of 55 μm to 130 μm, and a ten-point average roughness (Rz) of 9 μm to 19 μm.
Patent Document 3 discloses a charge roller as another example of the above techniques. The charge roller includes a conductive support, a semi-conducting elastic layer in the roller form provided on the conductive support, and a protective layer formed on the semi-conducting elastic layer. The protective layer is formed by applying a coating liquid for forming the protective layer, the coating liquid containing microparticles exhibiting a function of preventing deposition of external material on the protective layer. The volume average particle size of the microparticles is regulated so that the surface roughness of the protective layer is adjusted to 1 μm or less.
According to the techniques disclosed in Patent Documents 1 to 3, uniformity in image density can be ensured by controlling the surface roughness of the top surface of the conductive roller by the action of microparticles incorporated into the coating layer. However, since in recent years image-forming apparatuses provide high-quality images at low current, generation of image unevenness cannot be prevented merely by controlling the surface roughness of the top surface of the conductive roller. In such a case, uniformity in image density fails to be ensured, which is problematic.